The invention is directed to a method for generating and storing digitized density thresholds for rastering a half-tone original image, particularly in the form of a color separation (plans). Density thresholds of a portion of a raster are stored in at least one data memory as memory words dependent on a spot function. A plurality of spots (raster points) in the portion that respectively cover a plurality of memory words are periodically arranged next to one another and above one another, forming a multiple reference cell (super cell). A sorted sequence based on a measure of the spot function is calculated for the memory words of the data memory, and density thresholds are allocated to the memory words of the data memory dependent on the position of the memory word in the sorted sequence.
The raster can thereby be turned relative to a scan direction of a recording means that scans a recording carrier along scan lines.
Limiting lines of the portion of the raster proceed in the scan direction as well as at a right angle relative thereto.
It is known for the rastered recording of tonal value signals (image signals) that are required by scanning the half-tone original image to superimpose these tonal value signals with density threshold signals (raster signals) of a raster that is turned relative to a recording direction (German Patent 19 01 101). The density threshold signals or, respectively, raster signals generated for this purpose correspond to a density structure content of a portion taken from the selected, turned raster whose limiting lines lie in the recording direction and in a feed direction that is orthogonal thereto. The basic period of the structure of the turned raster with respect to each of the two orthogonal directions is contained once in the portion. The portion thereby comprises a plurality of raster points (spots) in scan direction as well as at a right angle thereto, these spots being periodically arranged next to one another and above one another. This portion can also be referred to as a multiple reference cell or super cell. As a consequence of the periodicity of the portion of the turned raster, the density threshold signals (raster signals) can be periodically repeated without further ado in order to be superimposed with the tonal value signals of larger image formats or, respectively, image portions. Over and above this, the raster portion can be resolved into so many sub-lines proceeding in the image recording direction that a plurality of such sub-lines devolve onto one image line width.
As a result of this subdivision of the raster portion, however, nothing regarding the size and periodicity thereof is changed. The condition that the raster portion seamlessly merges into itself in each of the two orthogonal directions (vertically and horizontally) is referred to as the wrap-around condition.
Data memories wherein the density thresholds of the raster portions are digitally stored are utilized for the implementation of the described, known method. The density thresholds stored therein, and imaged visually, can also be referred to as density hills or, respectively, threshold hills. The density thresholds are stored in the data memories in such a form that the required raster information are input by previous scanning of a model raster and quantizing as well as coding of the raster signals. The digital density thresholds thus contained in the data memories serve the purpose of being converted back into analog values with the tonal value signals scanned line-by-line from the original image and of being subsequently supplied into superimposition and threshold units. Typically, round, oval or rectangular point forms that grow proceeding from the middle are employed when scanning an optical original.
Digital recorders are also used in order to expose the individual color screen separations, a light source, particularly a laser beam, for exposing a light-sensitive surface therein being capable of being displaced with constant step size in two orthogonal directions. The light source is thereby switched on or off in clocked fashion in order to expose or not expose small picture elements. These picture elements are referred to as dots or pixels. Since a high resolution of the recorder is desirable, the memory requirement for storing the portion of the raster structure wherein the density thresholds are contained becomes correspondingly high. Since a raster point or spot in digital recorders is constructed by a plurality of neighboring pixels, all the more gray scales can be realized as there are pixels available for constructing the spot. For converting an original image into the pixels to be exposed, the digitally working recorder is preceded by a known means referred to as a raster image processor wherein user preset values such as raster width, raster angle and gray scale values are converted in the generation of the signals stored bit-by-bit for the pixels to be exposed. In this means, thus, a pixel is represented by one bit in a memory organized in bit fashion whose totality is referred to as a bit map. In a data memory of the means, at least one raster point or spot of a raster portion is presented by data words that represent the density thresholds. This memory content, that is referred to as a reference cell or as a reference cell memory as well, has a width of m words and a height of n words. Width and height of the reference cell are also referred to as columns and rows. A word of the reference cell is allocated to every pixel of the bit map. The numerical values of the words of the reference cell, which represent the density thresholds, thus define the sequence in which the bits are to be placed for increasingly darker gray. The density thresholds in a reference cell are defined by a prescribed spot function, so that a raster point or spot can assume many forms. The content of the reference cell is worked off when, for digitized rastering of the half-tone original image, the tonal value signals thereof are compared to the density thresholds and, dependent on the result of the comparison, a bit which represents the status of a pixel and which is part of a bit map in the memory structured in bit fashion is set or not. As a consequence of its periodicity, the handling of the reference cell can simply occur such that, for example, the density thresholds along a row are interrogated and, when the right-hand edge is reached, a new placement is made in the same row at the start of the reference cell.
Given turned rasters, when the raster angle is unequal to zero, the corner points of a spot must lie on rational pixel coordinates in order to preserve the periodicity. The edges of the spot can thereby be limited in step-shaped fashion. Large reference cells having a corresponding memory requirement are thereby necessary for producing the periodicity or wrap-around condition.
The problem as to how the density thresholds to be stored are expediently formed in detail in order to reproduce a half-tone original image in such rastered fashion that the half-tone image acts uniformly or, respectively, "at rest" over a sub-area that has the same tonal value is thereby not treated for that instance wherein the thresholds are not digitally formed by scanning an optical original but are fully digitally formed according to the measure of a two-dimensional function. This function is also referred to as spot function. For the full digital generation of the density thresholds of the raster points of a multiple reference cell (super cell) in the prior art known from practice, the total number of memory words of the multiple reference cell is first determined. The determination of the total number can occur based on the measure of the raster angle, of the raster width, and according to the resolution of the system. A sorted sequence of memory words of the multiple reference cell is then formed based on the measure of the spot function. Density thresholds are then allocated to the memory words of the multiple reference cell in linear dependency on their position in the sorted sequence. In fact, the individual raster points (spots) of a multiple reference cell (super cell) differ somewhat insofar as the plurality of memory words allocated per respectively one of the raster points fluctuates at the individual raster points and usually does not correspond to the rated value calculated from the total plurality of memory words of the multiple reference cell and from the plurality of the raster points or subcells. This results therein that more or fewer pixels are set in neighboring raster points when, based on the measure of a specific gray tint, a defined fraction of all of the pixels allocated to the multiple reference cell is to be blackened. The impression of black points of different size on a white ground thereby arises for the viewer of the reproduced half-tone original image, at least when the gray scale value amounts to substantially less than 50%. When, by contrast, a darker gray tint that lies considerably above 50%, for example at 70%, is reproduced upon employment of a multiple reference cell, then the impression of light spots of different size on a black ground arises here for reasons that are similar to those set forth above for low gray scale values. In both instances, the reproduced half-tone image makes a restless impression.
It is therefore expedient to undertake a correction of gray scale value with which a half-tone image original is rastered such that the impression of the half-tone image reproduced according to it is uniform or, respectively, at rest. For correction of gray scale value, an average number of memory words of the spot of the portion of the raster can preferably be calculated and can be compared to the actual number of memory words of respectively one spot. Dependent on the result of the comparison, the density thresholds to be allocated are raised for a comparatively large number of memory words for positions at the start of the sorted sequence of memory words of the spot. This means that the plurality of pixels to be blackened for low gray scale values when recording is reduced, and this can be referred to as under-drive. Given a relatively low plurality of memory words of the spot, the density thresholds to be allocated are lowered, as a result whereof the plurality of pixels to be blackened for low gray scale values is increased, and this can be referred to as over-drive. Given a comparatively large plurality of memory words for positions at the end of the sorted sequence of memory words of the spots, the density thresholds to be allocated are lowered and, as a result thereof, the plurality of pixels to be blackened for high gray scale values is super elevated, i.e. over-driven. Given a relatively low plurality of memory words of the spot, the density thresholds to be allocated are raised and, as a result thereof, the plurality of pixels to be blackened for high gray scale values is lowered, i.e. under-driven.
When, as is standard, the spots (raster points) grow proceeding from the middle, white spots are concentrated in the corners of the respective raster point given dark tints. As a result thereof, disturbing, different sizes of white points between the blackened raster points can occur. The reproduced image can therefore make a restless impression. This effect can be eliminated with a correction of white value, this being expediently comprised therein that every spot is subdivided into four quadrants and, given relatively high gray scale values (greater than 50%), a square (spot) subject to the correction of gray scale value is composed of respectively one quadrant of four neighboring spots.
The corrections of gray scale value and white value generally yield good results. In individual cases, disturbing patterns can still derive, particularly in the ranges of the raster angles of 0 .+-.2 degrees and 45 .+-.2 degrees. Since the spots are arranged grid-like, the possibility of the formation of grid lines exists. The problem of a different point closure arises as a further cause of a restless image in the ranges of the raster angles from 0 through 35 2 degrees and 45 through .+-.2 degrees. What is to be understood by this is that neighboring raster points touch with increasing gray scale value beginning with a certain gray scale value. The point closure occurs at a different gray scale value for specific positions of the spot boundaries. This disturbance also tends to the formation of grid lines.